Time Domain Reflectometry (TDR) is an established method for determining the propagation velocity of Radio Frequency (RF) pulses along transmission lines, or probes that are typically inserted into the target material undergoing testing, to calculate water or other liquid content, determine content/volume levels in a container, or to determine material dielectric constants.
As is well known to those skilled in the art, TDR apparatus and methods have been widely used in a wide range of applications measuring water/liquid content based upon the principle that the dielectric constant (K) (i.e. the measure of relative permittivity) of water is approximately 76.9 at 23° C. while the dielectric constants for numerous other known materials are considerably lower. Utilising this disparity in permittivity provides an excellent means of measuring a material's water-content or other dielectric characteristic. The apparent dielectric constant of a moist material sample is a function of the propagation velocity of an electromagnetic wave as it transits an RF transmission line. Accurate detection of the instant the pulse starts travelling along the transmission line and the point of reflection of the pulse from the terminal end of the transmission line allows determination of the propagation velocity, and thus permittivity, of the surrounding material.
A characteristic amplitude discontinuity of the RF signal is generated at both the initiation of the pulse exiting the transmission line mounting and the point of reflection of the pulse at the end of the transmission line. However, due to the very short transit time (approximately 1 nanosecond) of the pulse between the start point of a typical 30 cm length of the transmission rod and the reflection point and the need for high accuracy levels to provide meaningful results, the issue of precisely determining both these instances is in itself a key parameter in the TDR systems' effectiveness and the subject of ongoing research.
Known TDR systems are often used invasively i.e. typically two conductive probes forming a waveguide are inserted into the target material to perform the moisture measurements. However, this has clear drawbacks in terms of operability and practicality. Consequently, non-invasive TDR systems have been used to avoid damaging the target material and to facilitate improved measurement cycle times. Non-invasive TDR systems also enable the transmission rods to be moved to different separations from the target material surface, thus generating different RF field geometries. The variation in geometries enables the deduction of moisture measurements relating to different sub-surface materials of differing dielectric constants. However, such movement needs to be precisely performed to avoid debilitating error in the target material moisture calculations. In practice, such movements of the transmission line have an attendant impact on the speed of measurements and the resulting accuracy.
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It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description, which is given by way of example only.